The greenhouse effect is defined as when the Earth's atmosphere becomes thick with gases and substances which trap the sun's radiation, making the Earth warmer.(noun)
An example of the greenhouse effect is global warming.
See greenhouse effect in Webster's New World College Dictionary
See greenhouse effect in American Heritage Dictionary 4
Energy radiated by the sun converts to heat when it reaches earth. Some heat is reflected back through the atmosphere, while some is absorbed by atmospheric gases and radiated back to earth.
Learn more about greenhouse effect